drm: Rename connector list iterator API

Currently the functions that initialize and tear down a connector
iterator use the _get() and _put() suffixes. However, these suffixes
are typically used by reference counting functions.

Make these function names a little more consistent by changing the
suffixes to _begin() and _end(), which is a fairly common pattern in
the rest of the Linux kernel.

Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170228144643.5668-8-thierry.reding@gmail.com